[asger-finding]

This PR

• changes the public-facing API
• is a bug fix

Describe the changes below:

PIXI.WebGLGraphics.graphicsDataPool was shared by all WebGL contexts, and switchMode popped from it without checking the context. So a Graphics in one game could get WebGLGraphicsData whose buffers belong to another game's context, or to a destroyed game's, and then fail every draw with the warning WebGL: INVALID_OPERATION: bindBuffer: object does not belong to this context.

Now each context has its own pool, set in WebGLRenderer#initContext (which also drops pooled buffers of a lost context on restore) and nulled in WebGLRenderer#destroy, alongside PIXI.glContexts and PIXI.instances.

This caused an extremely rare but crazy bug in our game. See below.

To reproduce:

1. Go to https://tanktrouble.com
2. Press 2 players and then Local game.
3. Paste this snippet in the Console:

const throwaway = new Phaser.Game({ width: 1, height: 1, renderer: Phaser.WEBGL, state: { create() {
    const shape = this.add.graphics().beginFill(0xffffff).drawRect(0, 0, 1, 1);
    setTimeout(() => shape.clear(), 100);
    setTimeout(() => throwaway.destroy(), 200);
} } });

4. Pick up a laser crate and hold an active laser. May require multiple rounds (its a random chance spawn).
   Crate:

5. Bug